From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 69CF1C4338F for ; Tue, 24 Aug 2021 15:38:54 +0000 (UTC) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 336B761151 for ; Tue, 24 Aug 2021 15:38:54 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.1 mail.kernel.org 336B761151 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=nongnu.org Received: from localhost ([::1]:53274 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1mIYVp-0002vE-88 for qemu-devel@archiver.kernel.org; Tue, 24 Aug 2021 11:38:53 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:43290) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mIYV0-00027v-Vn for qemu-devel@nongnu.org; Tue, 24 Aug 2021 11:38:02 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:38097) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1mIYUz-0005gg-JI for qemu-devel@nongnu.org; Tue, 24 Aug 2021 11:38:02 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1629819480; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=6JfRZp7+4iDWqXYzu/J3+zBKraFtOmvMRPWPk+mlZOw=; b=MwLipXQbdvZqD3fYekmiaLprkCOXfiAaCUwmEMxfgP3GKs2Hfl3Dz1w1oCHRpzp1U8w1dC O6Xxu+Vs3S//6tXw+qJiP+4VlzjX8MndB9D6rPny8pVz2tWLOIQG5XkOWZM9NHv+68OKBM UZP1RbqKvKkmNuF5srmjLSjxj43u+lI= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-114-S0L-MH36MjSPOnh7D7vXPw-1; Tue, 24 Aug 2021 11:37:57 -0400 X-MC-Unique: S0L-MH36MjSPOnh7D7vXPw-1 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 5A1C58799ED; Tue, 24 Aug 2021 15:37:56 +0000 (UTC) Received: from redhat.com (unknown [10.39.192.114]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 8C8835D9C6; Tue, 24 Aug 2021 15:37:55 +0000 (UTC) Date: Tue, 24 Aug 2021 17:37:53 +0200 From: Kevin Wolf To: Christopher Pereira Subject: Re: qcow2 perfomance: read-only IO on the guest generates high write IO on the host Message-ID: References: <55980bc8-97ad-77a4-1bb7-a086f2712ea1@imatronix.cl> MIME-Version: 1.0 In-Reply-To: <55980bc8-97ad-77a4-1bb7-a086f2712ea1@imatronix.cl> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=kwolf@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Received-SPF: pass client-ip=170.10.133.124; envelope-from=kwolf@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -34 X-Spam_score: -3.5 X-Spam_bar: --- X-Spam_report: (-3.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.747, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=unavailable autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: qemu-devel@nongnu.org, qemu-block@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" [ Cc: qemu-block ] Am 11.08.2021 um 13:36 hat Christopher Pereira geschrieben: > Hi, > > I'm reading a directory with 5.000.000 files (2,4 GB) inside a guest using > "find | grep -c". > > On the host I saw high write IO (40 MB/s !) during over 1 hour using > virt-top. > > I later repeated the read-only operation inside the guest and no additional > data was written on the host. The operation took only some seconds. > > I believe QEMU was creating some kind of cache or metadata map the first > time I accessed the inodes. No, at least in theory, QEMU shouldn't allocate anything when you're just reading. Are you sure that this isn't activity coming from your guest OS? > But I wonder why the cache or metadata map wasn't available the first time > and why QEMU had to recreate it? > > The VM has "compressed base <- snap 1" and base was converted without > prealloc. > > Is it because we created the base using convert without metadata prealloc > and so the metadata map got lost? > > I will do some experiments soon using convert + metadata prealloc and > probably find out myself, but I will happy to read your comments and gain > some additional insights. > If it the problem persists, I would try again without compression. What were the results of your experiments? Is the behaviour related to any of these options? > Additional info: > > * Guest fs is xfs. > * (I believe the snapshot didn't significantly increase in size, but I > would need to double check) > * This is a production host with old QEMU emulator version 2.3.0 > (qemu-kvm-ev-2.3.0-31.el7_2.10.1) Discussing the most recent version is generally easier, but the expected behaviour has always been the same, so it probably doesn't matter much in this case. Kevin